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Glossary B
In networking, what are bandwidth and latency? Bandwidth and latency are the two main factors governing the performance of a network. Bandwidth, typically measured in bits per second, is the rate at which data flows over the network. Latency is the time that elapses between a request for information and its arrival. Bandwidth is a measure of capacity rather than speed, but just as more water flows through a wide river than a small creek, high bandwidths generally result in faster networks. Unfortunately, a high latency can tremendously degrade the performance of even the largest capacity network. Because the speed of light is finite, there will always be some latency present, but slow servers, inefficient data packing, and excessive network hopping can add up to a lot of transmission delay. Also, while most vendors advertise the theoretical bandwidths of their products, due to bottlenecks, hardware problems, and high loads, the effective bandwidth will usually be substantially less. For a scorecard of the bandwidths and other characteristics of a number of networking technologies, see: http://www.ee.siue.edu/~bnoble/comp/networks/docs/lan-technology.html The above document doesn't address latencies, but modems typically have values of around 100 milliseconds, vastly higher than any of the technologies reviewed on the scorecard. For a good overview of latency and its effects, read Stuart Cheshire's It's the Latency, Stupid, at: http://www.stuartcheshire.org/rants/Latency.html Also see:
What are bitmap and vector graphics and how are they different? A bitmap (also called "raster") graphic is created from rows of different colored pixels that together form an image. In their simplest form, bitmaps have only two colors, with each pixel being either black or white. With increasing complexity, an image can include more colors; photograph-quality images may have millions. Examples of bitmap graphic formats include GIF, JPEG, PNG, TIFF, XBM, MacPaint, BMP, and PCX as well as bitmap (i.e., screen) fonts. The image displayed on a computer monitor is also a bitmap, as are the outputs of printers, scanners, and similar devices. They are created using paint programs like Adobe Photoshop. Vector (also known as "object-oriented") graphics are constructed using mathematical formulas describing shapes, colors, and placement. Rather than a grid of pixels, a vector graphic consists of shapes, curves, lines, and text which together make a picture. While a bitmap image contains information about the color of each pixel, a vector graphic contains instructions about where to place each of the components. It is even possible to embed a bitmap graphic within a vector graphic, which is how vector-bitmap hybrid graphics work. It is not possible, however, to embed vector information within a bitmap. Examples of vector graphic formats are PICT, EPS, and WMF as well as PostScript and TrueType fonts. These are created with GIS and CAD applications as well as drawing programs like FreeHand. SVG, or Scalable Vector Graphics, is a language for describing vector graphics in XML. With SVG, you can code graphics directly into an XML document. For more information about SVG, see: http://www.wdvl.com/Authoring/Languages/XML/SVG/ As described below, bitmap and vector graphics both have their strengths and weaknesses:
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What is a binary file? A binary file is one in which the eighth bit of each byte is used for data. Computers and programs can read binary files, but people cannot. Executable files, compiled programs, Microsoft Word documents, SAS and SPSS system files, and spreadsheets are all examples of binary files. Files that contain machine-specific codes (i.e., processor-specific microcode) are binary files. However, not all binary files contain processor-specific codes. Some binary files contain text or data in a non-ASCII format that is unrelated to the microcode used by the processor. For example, most graphics files, all compressed files, and many other file types use all eight bits per byte, so are called binary. Also see:
What is a bit? A bit is a binary digit, the smallest increment of data on a computer. A bit can hold only one of two values: 0 or 1, corresponding to the electrical values of off or on, respectively. Because bits are so small, you rarely work with information one bit at a time. Bits are usually assembled into a group of eight to form a byte. A byte contains enough information to store a character, like "h". Also see: What is a block? A block is the smallest logically addressable unit of data that a specified device can transfer in an input/output operation (512 or 1024 contiguous bytes for most disk devices). Also see: What is a broadcast or data storm? A broadcast or data storm is excessive transmission of broadcast traffic in a network. This happens when a broadcast across a network results in even more responses, and each response results in still more responses, in a snowball effect. If network traffic reaches near 100 percent of the available bandwidth, all network traffic can be blocked. Broadcast storms are often caused by a defective network adapter or defective cabling, where the card or cable floods the network with packets. This can be fixed quickly by disconnecting the computer system from the network and then replacing the Ethernet card, or by checking the network cable for any breaks, kinks, or loosened connectors, and then making repairs as needed. A broadcast storm can prevent access to server resources, or cause an entire network to go down. Broadcast and data storms can also be caused by an intentional attack with the purpose of bringing down network systems. When this is the case, they are called distributed denial of service (DDoS) attacks. To prevent your computer from being a victim of or participant in a DDoS attack, make sure you have installed the latest security patches and have all your software up to date. For more information about DDoS attacks and defense against them, visit the following URLs: http://searchsecurity.techtarget.com/bestWebLinks/0,289521,sid14_tax281936,00.html
http://www.opensourcefirewall.com/ddos_whitepaper_copy.html Also see:
What is a buffer? A buffer is an internal memory area used for temporary storage of data during input or output operations. This storage area is usually in RAM. A buffer can be used in different ways. Most programs (e.g., word processors, graphics programs) keep track of changes in the buffer and then copy the buffer to a disk. Buffers are also used for printing. When you print, your computer copies the document to a print buffer. The printer runs in the background while your computer performs other tasks. This is called spooling. Also see:
What is a byte? "Byte" is an abbreviation for "binary term". A single byte contains eight consecutive bits, and is capable of storing a single character. Also see: What was BITNET, and what happened to it? Based on IBM's VNET, which the company used for internal communications, the Because It's Time NETwork, or BITNET, was launched in 1981. It first connected Yale to CUNY, but throughout the 1980s, BITNET experienced rapid growth, reaching thousands of computers in the US and Mexico. Sister networks, physically part of BITNET but governed by different bodies, formed in Canada (NetNorth), Europe (European Academic and Research Network, or EARN), Japan (AsiaNet), and other locales. It became the preeminent network for universities and research institutions worldwide. Through it, users could exchange electronic mail, files, and interactive messages between member institutions. A number of today's popular technologies, including LISTSERV mailing lists, originated on BITNET. Compared to the Internet, BITNET was not robust, nor did it have much bandwidth. To reduce costs, network traffic passed between two institutions by just one path. Partially making up for this lack of redundancy was BITNET's foundation as a store-and-forward network. As it passed to its destination, data was often relayed through several institutions or nodes. Each node would temporarily store this data before forwarding it to the next. If there was a temporary break in the network preventing a node from forwarding its traffic, it could hold the data until the network became available again. As the Internet grew and became more affordable, it became apparent that BITNET was obsolete. In 1996, BITNET's managing body, the Corporation for Research and Educational Networking (CREN), recommended that its members terminate their use of BITNET by the end of that year. Today, BITNET in its original form is largely defunct. However, BITNET II, which uses the Internet as a medium to transfer BITNET protocols, is still in use by some institutions. Also see:
What is bus mastering? Bus mastering is a bus design that allows a peripheral (plug-in board) to access the computer's memory independently of the CPU. This allows data transfer between the peripheral and the main system memory while the CPU is being used by other devices. Bus mastering usually requires that the adapter or peripheral have its own built-in processor so that it can operate independently of the CPU. A bus-mastering peripheral can control the bus and act as if it were the entire computer system. The most common bus mastering devices are hard drive controllers (e.g., IDE, SCSI), video adapters, and network interface cards. Bus mastering can function independently of the bus architecture. Bus mastering adapters can be ISA, EISA, VLB, PCI, AGP, USB, or FireWire. Currently, most bus mastering on PCs is done on the PCI bus. In addition, support has been added for IDE/ATA hard disk drives to do bus mastering on PCI under certain conditions. In theory, bus mastering can allow the processor to do other work simultaneously, but in practice, there can be some complications. In order to do bus mastering properly, a facility to arbitrate between requests to "take over the bus" must exist. The system chipset handles these requests. Bus mastering is also known as "first party" DMA (Direct Memory Access), since the device doing the transfer controls the work. For additional information on bus mastering, please see: http://www.pcguide.com/ref/mbsys/buses/func_Mastering.htm Also see: |
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